The interior of a bulk superconductor cannot be penetrated by a weak magnetic field, a phenomenon known as the Meissner effect.
In type-I superconductors, superconductivity is abruptly destroyed via a first order phase transition when the strength of the applied field rises above a critical value Hc.
[1] and BeAu [2] The covalent superconductor SiC:B, silicon carbide heavily doped with boron, is also type-I.
This state, first described by Lev Landau, is a phase separation into macroscopic non-superconducting and superconducting domains forming a Husimi Q representation.
The ratio of the London penetration depth λ to the superconducting coherence length ξ determines whether a superconductor is type-I or type-II.